The present invention relates to an optical recording medium which is capable of recording and reproducing and erasing information at a high density and in a large capacity by means of laser beam.
An information recording medium using laser beams for reading information or erasing recorded information by means of laser beams is provided with a reflecting metallic film formed in the rear surface thereof. The recording medium includes a video disk or a compact disk dedicated for an information reproduction; an information-adding type disk; a re-writable phase change type disk; a magnetic optical disk. For example, as shown in FIG. 5, a re-writable phase change type disk is made of a transparent substrate 1 consisting of polycarbonate having a laser beam guiding groove formed therein, a first dielectric layer 2 formed on the surface on which the groove of the transparent substrate 1 is formed, a recording film 3 formed on the first dielectric layer 2, a second dielectric layer 4 formed on the recording film 3, and a reflecting film 5 formed on the second dielectric layer 4, an adhesive layer 6 formed on the reflecting film 5, and a protecting plate 7 or another disk adhered to the adhesive layer 6. Normally, vacuum evaporation or sputtering is used to form the first and second dielectric layers 2 and 4, the recording film 3, and the reflecting layer 5. The reflecting film 5 is made of aluminum or aluminum alloy, which is inexpensive and has a high reflection factor.
Laser beams for recording, erasing, and reproducing information are incident on the substrate 1 in the direction shown by arrows (A). In order to do the recording, erasing, and reproducing operations, the intensity of laser beams are varied according to information which is recorded, erased or reproduced and the reflected beams are detected. A reflecting layer made of aluminum deteriorates in its optical performance. That is, an information-adding type or re-writable optical recording medium greatly deteriorates in its reading and writing performance. The reason is as follows: the corrosion of the reflecting layer 5 made of aluminum causes the reflecting layer 5 to become irregular, and consequently, the reflection factor of the reflecting layer 5 is reduced. The corrosion-resisting performance of the reflecting layer 5 may be improved to some extent by using aluminum of high purity, but crystal growth cannot be avoided. An additive may be mixed with aluminum powder to improve corrosion-resisting performance of the reflecting layer 5, but it cannot be improved as desired.